1. Field of Invention
The present invention relates to a switching power supply. More particularly, the present invention relates to the pulse width modulation (PWM) of the switching power supply.
2. Background of the Invention
The PWM controller is an integrated circuit used in the switching power supply to control and regulate the switching duty cycle. Because of environmental regulations, the power of the computer and other electrical products have been required to meet the power management and energy conservation standards. The power management is to manage the system to consume power only during operation, with only very little power being consumed during the non-operation mode. With respect to the power supply in a power management application, how to save the power in the unloaded condition is a major concern. According to the invention, the object of the adaptive off-time modulation for the PWM control is to optimize the saving of the power consumption in lightly loaded and unload conditions.
FIG. 1 shows a traditional flyback circuit of the power supply, in which a PWM controller 100 controls the power output and achieves the regulation. The operation of PWM-control starts on the charging of capacitor 220 via a start-up resistor 210 when the power supply is turned on until the voltage VCC reaches the start-threshold voltage. Then the PWM controller 100 starts to output the PWM signal and drive the entire power supply. After the start-up, the supply voltage VCC is provided from the auxiliary bias winding of the transformer 400 through a rectifier 230. The resistor 240 converts the switching current of the transformer 400 to the voltage for the PWM control and over-power protection. If the auxiliary bias winding of transformer 400 cannot provide sufficient power for the supply voltage VCC the PWM controller 100 will be turned off as long as the supply voltage VCC is lower than the stop-threshold voltage. The feedback signal VFB is derived from the output of an optical-coupler 250. Through a resistor 290 and a zener diode 280, the input of the optical-coupler 250 is connected to the output of the power supply Vo to form the feedback loop. Through the control of the feedback loop, the,voltage of the VFB controls the on-time Ton of the PWM signal through the PWM controller 100 and dominantly decides the output power.
Concerning the loss of the power supply for the lightly loaded condition, the major losses are proportional to the switching frequency F. The major losses include the transformer core loss, the transistor switching loss, and the snubber power loss, for example. The switching period T is the inverse of the switching frequency, T=1/F. The increase of the switching period T reduces the power loss. However, the switching frequency is restricted by a short on-time Ton of the switching period for the transformer and the power supply with size shrinkage. To prevent the saturation of the transformer, a shorter Ton is required. The power consumption of the power supply is reduced in response to the increase of switching period T in the lightly loaded and unloaded conditions. Nevertheless, it is unsafe to increase the switching period without the limitation of Ton since the transformer may saturate due to the extended Ton. The saturation of magnetic components, such as inductors and transformers, causes over-stress damage to the switching devices such as transistors and rectifiers. Although the decrease of the switching frequency will reduce the power consumption in the lightly loaded and unloaded conditions, it might create an audio noise if the switching frequency falls into the audio band. According to this invention, another object of the adaptive off-time modulation for the PWM control is to protect the magnetic components from being saturated and to reduce the acoustic noise when the switching frequency is decreased in the lightly loaded and unloaded conditions.
Varying the switching frequency and/or entering the xe2x80x9cpulse-skippingxe2x80x9d mode in accordance with the loaded condition have been disclosed to increase the regulator efficiency in prior art. For example, U.S. Pat. No. 6,100,675, xe2x80x9cSWITCHING REGULATOR CAPABLE OF INCREASING REGULATOR EFFICIENCY UNDER LIGHT LOADxe2x80x9d (incorporated herein by reference) disclosed an oscillation frequency control circuit capable of varying an oscillation frequency of the oscillator circuit in response to loaded conditions. Another method is disclosed in U.S. Pat. No. 6,366,070 B1, xe2x80x9cSWITCHING VOLTAGE REGULATOR WITH DUAL MODULATION CONTROL SCHEMExe2x80x9d, which disclosed the regulator employing three operation modes. In this patent, the switching components are operated at constant frequency for heavy loads, the dual modulation control scheme is used for moderate to light loads, and the regulator enters a xe2x80x9cpulse-skippingxe2x80x9d mode for light loads. The disadvantages of foregoing prior art are: (1) Varying the switching frequency without the limitation of maximum switching on-time may result in saturation of magnetic components and cause over-stress damage to the switching devices such as transistors and rectifiers; (2) The modulation of switching frequency is only controlled by the load condition, and there is no correlation with the supply voltage. In order to save more power in the lightly loaded and unloaded conditions, the switching frequency has to be reduced as low as possible. However, if the switching frequency is reduced too low, the auxiliary bias winding of the transformer or inductor might be unable to provide sufficient power for the supply voltage of the PWM controller. It may cause the PWM control to work improperly. To achieve the best power saving performance, correlating the frequency modulation with both load condition and supply voltage is necessary. (3) In the lightly loaded and unloaded conditions, the switching frequency might be decreased to the audio band (such as 200 Hz 8 KHz). If the magnetic components are not well impregnated, the audio band switching frequency might generate an undesirable acoustic noise.
Thus, there exists a need for a reliable, stable and noiseless apparatus for improving the efficiency and saving the power consumption in lightly loaded and unloaded conditions, while avoiding the shortcomings of prior art.
The above-referenced deficiencies in the prior art are addressed in the present invention, which provides a reliable, stable and noiseless method and apparatus.
The invention provides an adaptive off-time modulation for a PWM controller to increase the switching period in the lightly loaded and unloaded conditions. The off-time modulation is accomplished by moderating a bias current of an oscillator in the PWM controller. Reducing the bias current increases the switching period, while the off-time of the switching period is extended. The maximum on-time of the PWM signal is kept as a constant. The feedback voltage that is derived from a voltage feedback loop, and the supply voltage of the PWM controller are taken as the variables to correlate with the off-time modulation. The bias current is modulated as a function of the feedback voltage and supply voltage. A threshold voltage is a constant that defines the level of the light load. A limit voltage defines the low-level of the supply voltage. A first differential signal is generated by subtracting the threshold voltage from the feedback voltage. A second differential signal is generated by subtracting the supply voltage from the limit voltage. The sum of the first differential signal and the second differential signal is converted into the bias current. A limiter clamps the bias current to set up the minimum switching period in normal loaded and fully loaded conditions. Once the feedback voltage is decreased lower than the threshold voltage, the bias current is reduced and the off-time of the switching period is increased continuously. When the supply voltage is lower than the limit voltage, the bias current is increased and a maximum off-time of the switching period is determined. Furthermore, the limit voltage is a variable of time, while the variation is found in every switching cycle. The maximum on-time and the off-time of the PWM signal determine the PWM frequency. Due to the off-time of the PWM signal varying in every switching cycle, the frequency spectrum of PWM signal is spread in lightly loaded and/or unloaded conditions; and therefore the acoustic noise is suppressed. The feedback voltage and the supply voltage determine the switching period of the PWM signal. Keeping the maximum on-time as the constant and increasing the switching period by increasing the off-time prevent the magnetic components, such as inductors and transformers, from being saturated.
Advantageously, the adaptive off-time modulation improves the efficiency and saves the power of the power supply in the light-load and no-load conditions. Meanwhile, spreading the frequency spectrum of the PWM signal reduces the acoustic noise.